MADISON — Wisconsin's gay marriage ban denies same-sex couples legal protections and serves no legitimate government purpose, the American Civil Liberties Union argued Monday in a brief asking a federal appeals court to preserve a ruling that struck down the prohibition.

The ACLU brought a lawsuit earlier this year on behalf of eight gay couples alleging the ban is unconstitutional. U.S. District Judge Barbara Crabb sided with the ACLU earlier this summer and ended the prohibition.

State attorneys under the direction of Republican Attorney General J.B. Van Hollen have asked the 7th Circuit Court of Appeals to reverse Crabb's ruling. The court has consolidated the case with a similar appeal from Indiana state attorneys; oral arguments are set for Aug. 26 in Chicago.

"By withholding from them the respect, recognition, and support that only marriage confers, Wisconsin stigmatizes these couples and their families as unworthy of the opportunity to express and legally embody their commitment in the most profound way that society provides," ACLU attorneys wrote in their brief.

Wisconsin voters amended the state constitution in 2006 to ban gay marriage or anything substantially similar. Wisconsin offers a domestic registry — which the state Supreme Court upheld as valid last week — that affords same-sex couples who join it a host of legal protections but gay couples contend the rights are limited.

The ACLU filed its lawsuit challenging the ban in February. Crabb ruled in June that the ban deprives gay couples of their due process and equal protection rights. More than 500 same-sex couples married in Wisconsin in the week between Crabb's decision and her order to halt the marriages pending Van Hollen's appeal.

Wisconsin Department of Justice attorneys filed a brief with the 7th Circuit late last month arguing there's no fundamental right to gay marriage. Crabb's ruling essentially created a new right that would dramatically expand federal authority into a realm traditionally controlled by the states, they said.

Sexual orientation isn't subject to special protection like gender or race and therefore doesn't deserve the same deference in equal protection claims, they argued. What's more, Wisconsin has historically defined marriage as a union between a man and woman, they said.

The ACLU countered in its brief that the state is withholding the full array of legal protections afforded to heterosexual couples, including the right to make medical decisions for one another, the right for both people to be considered parents of one of their children and the right to joint adoption.

All people enjoy a fundamental right to marry, including same-sex couples, and courts don't have to perpetuate historical limitations on rights once they're recognized as fundamental, the brief said.

The ban serves no rational government interest and is intended to thwart same-sex couples' aspirations and put them at a disadvantage, the ACLU argued.

Gay marriage is legal in 19 states as well as the District of Columbia. Gay marriage advocates have won more than 20 court victories across the rest of the country since the U.S. Supreme Court struck down a portion of the federal Defense of Marriage Act that prohibited the federal government from recognizing same-sex marriage in 2013.